[tomjakubowski]

Header-only libraries sometimes require you to define those externs in exactly one "impl" file, which you compile and link to your artifact. Something like this:

    // libfoo_impl.c
    // or in some other translation unit, such as main.c
    #define LIBFOO_IMPL
    #include "vendor/libfoo.h"

    // main.c
    #include "vendor/libfoo.h"
    
    int main() {
      foo_inc();
      printf("%d\n", foo_count);
      return 0;
    }

    // vendor/libfoo.h  
    #pragma once
    extern int foo_count;
    void foo_inc() {
      foo_count += 1;
    }

    #ifdef LIBFOO_IMPL
    int foo_count = 0;
    #endif

main.c and libfoo_impl.c both include libfoo.h, which declares foo_count with the right linkage, but the variable is only defined once, in whichever translation unit defines LIBFOO_IMPL.

Occasionally you'll find a header library which supports this _IMPL paradigm as an option to avoid inlining its functions in every translation unit that calls them.